SEOUL, South Korea — North Korean leader Kim Jong Un’s sister visited frontline areas where soldiers are reinforcing the heavily armed border with South Korea and renewed a threat to retaliate against any South Korean provocation targeting the fortification work, state media said Wednesday.

Tensions between the war-divided rivals have risen since three South Korean soldiers were injured last month in land mine blasts that the South’s military blamed on North Korean devices planted south of the countries’ military demarcation line.

Pyongyang’s official Korean Central News Agency said Kim Yo Jong toured the areas Tuesday with Korean People’s Army Marshal Pak Jong Chon, one of her brother’s top military officials. She called the North’s fortification efforts a “sacred and great undertaking” supporting the leader’s policy of permanently severing ties with South Korea, which he has declared his most hostile adversary amid a broader stalemate in diplomacy with Washington.

“Our military leadership underlined the need to make those rogues aware that our new firepower units have taken their firing positions,” she said, while reminding soldiers of her brother’s order to make the border with the “enemy … eternally impregnable.”

Following an inspection with the U.S.-led United Nations Command, Seoul earlier this month formally blamed North Korean-planted mines for the Sept. 21 border blasts and urged the North to apologize and clear its remaining mines. That announcement came after North Korea denied responsibility for the explosions and warned of strong retaliation if provoked.

Kim Yo Jong’s border inspection came a day after she warned that the North could restart anti-Seoul propaganda broadcasts with loudspeakers, which were a previously used psychological warfare tool, and could deploy more powerful weapons along the border with South Korea. She accused the South of firing warning shots and making warning broadcasts toward North Korean soldiers fortifying border areas under a false pretext that they crossed the border, and insisted that such provocations have increased “beyond the red line” since the mine blasts.

South Korea’s Joint Chiefs of Staff chairman, Jin Yong-Sung, called Kim Yo Jong’s threat “intolerable” and ordered the military to “sternly respond” to potential provocations by the North.

North Korea in recent years has added mines, barbed wire fences, anti-tank barriers and other fortifications to what’s already one of the world’s most heavily armed borders as Kim Jong Un pushed a hard line toward Seoul.

South Korea’s military has said North Korean soldiers crossed the border’s military demarcation line 50 times this year during fortification work, and that it responded with warning shots and broadcasts.

During Tuesday’s border inspection, Pak accused the South of making “deliberate, premeditated and absurd” claims to shift the blame to the North over the rising tensions and threatened an “overwhelming response” to South Korean provocations. He denied that the North Korean soldiers were breaching the border during fortification work, saying “our army knows where our borderline is better than any others,” KCNA said.